The present invention relates to means for effecting temperature-compensation of focal length in an optical assembly.
Certain optical assemblies such as those found in laser beam output scanners require a stable monochromatic collimated light beam provided by a beam source, such as a laser diode, and a collimating lens. For adequate optical performance, the beam source must maintain a predetermined beam quality over a wide ambient temperature range. The relative position between the collimating lens and beam source, known as the back focal length, must be maintained over a substantial range of ambient temperature variation. The allowable variance of this back focal length depends on the resolution of the scanner. The successful implementation of a very high resolution (e.g., 1,000 dpi) laser beam output scanner may, for example, tolerate no more than 20 microinches of back focal length deviation.
In conventional approaches to the problem, the beam source and lens are mounted in a mechanical structure that attempts to maintain the back focal length while the apparatus undergoes temperature-induced structural changes. Hence, the athermalization (i.e., thermal compensation) of the beam source is effected either passively or actively.
Passive compensation systems typically rely on the differences in coefficients of thermal expansions of the various elements in the optical system such that there is minimal net focus shift with temperature. The conventional approach is to employ concentric tube systems, which, if constructed from common materials, are too large or bulky. For example, U.S. Pat. No. 4,730,335 discloses a series of interlocking tubes each carrying a single optical element of an optically-pumped solid-state laser. However, the accuracy of a passive system will depend upon the CTE of the materials chosen for the construction of certain components of the system. Thus, some passive designs cannot be manufactured because the desired CTE is simply not exhibited by the materials suited for fabricating the requisite system components, or because the desired CTE is available but only in materials that are unstable, thus rendering a system response that is inaccurate, unstable, or unreliable.
Active compensation systems typically rely on active temperature control of the beam source such that the beam source temperature is made constant. For example, a thermoelectric cooler is employed in the apparatus disclosed in U.S. Pat. No. 4,604,753 to stabilize the output power and wavelength of a laser diode beam source; U.S. Pat. Nos. 4,656,635 and 4,993,801 disclose a beam source wherein a thermoelectric cooler is employed to control the operating temperature of the entire optical head. However, as will be described below, conventional active athermalization approaches have been found less than desirable.